Ungrounded power line sensors measure the voltage of a medium-voltage power line relative to ground through, for example, a capacitive coupling between metal plates on the outside of the sensor and ground. Under normal conditions, accurate voltage measurements are possible. However, the accumulation of snow and/or ice on the sensor can change the capacitive coupling between the sensor and ground, resulting in errors in the measurement of line voltage.
Snow and ice effects are thus a problem with this type of capacitive voltage sensor. For example, U.S. Pat. No. 4,795,973 describes a modification of the system described in U.S. Pat. No. 4,689,752 with the objective of being less sensitive to snow. U.S. Pat. No. 4,795,973 describes a configuration where the entire sensor body is turned into a single, large voltage sensing plate. All of these patents are incorporated herein by this reference. These approaches may still be somewhat sensitive to snow because significant snow build-up will change the effective surface area of the sensor, instead spuriously increasing the voltage rather than spuriously decreasing the measured voltage.
In the prior art, no known entirely successful mitigation technique has been found for voltage measurement error due to snow and/or ice accumulation for sensors without a neutral connection.
To be completely resistant to the effects of snow and ice, one typical solution is to use relatively large and heavy instrumentation transformers wired directly to each phase. A “Potential Transformer” (PT) is used to transform the line voltage down to a lower voltage that is more easily measured, typically about 120 Vrms. By measuring this lower voltage and multiplying by the turns ratio of the PT, the line-to-neutral voltage of a phase can be deduced. A “Current Transformer” (CT) is used to measure current. The line to be monitored passes once through a transformer core. A secondary with many turns is also wound around the transformer core, and the secondary is either shorted or drives a very small resistance. The secondary is isolated from the voltage on the primary, and the current on the secondary is much lower than (and proportional to) the current on the line, with the turns ratio of the transformer again being the proportionality constant. Voltage, current, power, etc., are then measured by a commercial meter attached to the PT and CT (for example, the ITRON Quantum Q 1000). Such a solution can be expensive and labor intensive to install.